Apparatus for accelerating charged particles, especially electrons, to very high-velocity



Jan. 8, 1952 w. E. wlLLsHAw 2,582,186

APPARATUS FOR ACCE LERATING CHARGED PARTICLES, ESPECIALLY ELECTRONS, TO VERY HIGHVELOCITY Filed Aug. 6, 1947 i Patented Jan. 8, 1952 APPARATUS FOR ACCELERATING CHARGED PARTICLES, ESPECIALLY ELECTRONS, T0 VERY HIGH-VELOCITY William Ernest Willshaw, Kenton, England, assignor to The General Electric Company, Limited, London, England Application August 6, 1947, Serial No. 766,757 In Great Britain November 14, 1945 Section 1, Public Law 690, August 8, 1946 Patent expires November 14, 1965 7 Claims.. (Cl. 315-6) This invention relates to apparatus for accelerating charged particles, such as electrons, or ions, and especially electrons. to very high velocities, and particularly to velocities approaching that of light.

In one known form of apparatus for accelerating charged particles, a stream of the particles is injected into an accelerating system consisting of a succession of cylindrical electrodes having their axes coaxial with the stream; these electrodes are connected alternately to one or other terminal of a source of high frequency potential so that particles passing down the cylinders at thecorrect velocity pass through a potential step which is always of the same sign when traversing the gaps between adjacent cylinders, so that the particles are progressively accelerated and leave the system having gained energy corresponding to the sum of the potential steps passed through.

With this known arrangement, in order to obtain the maximum acceleration in the shortest length it is desirable that the voltage step at each gap should be as large as possible and a transformer is` therefore usually used at the voltage source to provide a high voltage for exciting the accelerating electrodes.

It is desirable for compactness to use for exciting the accelerating electrodes oscillations of high frequency, but the difliculties of connecting together the accelerating electrodes, and also the difliculties of designing suitable electrodefeeding transformers, increases rapidly with frequency,V and the maximum frequency useable with this known form of apparatus is limited by these considerations; the apparatus has therefore not been used for accelerating electrons` which, in view of their high velocities, would require the use of very high frequencies to obtain apparatus of practicable dimensions.

, The object of this invention is to provide an alternative form of apparatus in which charged particles are similarly progressively accelerated and which may be designed for accelerating electrons.

According to the invention apparatus for accelerating charged particles such as electrons or ions comprises means for producing a stream of the charged particles, a plurality of hollow resonators spaced along the path of said particles and each arranged to produce when excited an oscillating electric Ifield capable in the appropriate part of the oscillatory cycle of accelerating particles of the stream, and means for exciting said resonators in such phase relationship that some particles of the stream are accelerated at each of the resonators in succession.

By a suitable design of resonator and ex-citing means a very high voltage step may be obtained across the parts of each resonator which are traversed by the particles, the resonator acting as a transformer of very high voltage ratio. As a result cf the high voltage steps which may be achieved in this way, and by using the very high frequencies which are practicable with this arrangement for exciting the resonators, a relatively compact system may be achieved, since the number of accelerating resonators required to achieve high particle velocity need not be very large and the distance between each may be small, being inversely proportional to the exciting frequency. .'Ihe latter may be arranged to be so high that the apparatus may readily be designed for accelerating electrons.

Each of the resonators may be arranged to be excited from a separate source, with suitable phase synchronisation between the sources, but preferably all the resonators are fed from a single high frequency source through a waveguide, allowance being made for the fact that the velocity of wave propagation along the guide is greater than the velocity of the particles in the stream; this may be effected, for example, by making the length of waveguide between successive resonators greater than the distance between the resonators along the stream i. e. by kinking the wave guide; alternatively, with cylindrical resonators whose resonant frequency is independent of their axial length, the axial length of the resonators may be arranged to increase progressively from the lower velocity end and the coupling points to the waveguide varied relative to the axial centres of the resonators so that the axial distances between the centres of successive resonators is less than the distances between the coupling points along the waveguide, and increases progressively.

For securing the highest frequencies of excitation of the resonators, which may be for example 3000 megacycles per second or higher for an electron accelerator, it will in general be preferable to excite the resonators intermittently, so as to produce bursts ofvery high velocity particles, and for this purpose there may be used one or more magnetrons pulse-operated at a suitably high repetitive rate, for example 500 pulses per second.

In order to stabilize the operating frequency, i. e. to reduce the possibility of mode-jumping,

3 of a pulse-operated magnetron which feeds a resonator via a waveguide in this way, it will in general be desirable to include attenuation in the waveguide andl it may be desirable to make the effective electrical length of the waveguide from the magnetron to the first resonator slightly diierent from the length which is these` retically the optimum for frequency stability" waveguide are preferably arranged tobespa'cedT apart at distances which are half-wavelengths;y or multiples thereof, of the wavelength intheguide corresponding to the required operating frequency.

The magnetron shouldfbe one capable of operation over a wide rangeof impedance varia-- tion without mode change, since though final operation is to a matched load impedance formedby the attenuating'waveguide and the resonators, the impedance presented to the magnetron in the initialstage ofthe building'up ofthe elds within the resonators at' each pulse may be veryreactive and different from the matched impedance.

The means for producing a stream of charged particles may be of any convenient form. Thus for producing a stream of'electrons a thermionic cathode may be arranged Within the rst accelerating resonator s'o that its emission is controlledlby the field set up within the resonator; but preferably the electron stream is produced by an electron gun arrangement, which may' b'e pulse-operated in synchronism with a pulseoperated source exciting the resonators, so that a stream of electrons is injected intothe first accelerating resonator atl a reasonably high speed, for example with about 50 kilovolts en-v ergy; this arrangement has the advantage that the electron stream may be initially focussed to counteract to some extent the spreading experienced by the stream in passing through the resonators; alternatively, or additionally, spe-r cial focussing means such as magnetic coils round the stream between the resonators or grids' in the path of the stream, may be used to pre= vent undue spreading of the stream.

In any case, the walls ofV the hollow resona' tors, or part thereof, may form part of the walls of a vacuum-tight enclosure in which the particles are adapted to move, or the resonators may be totally enclosed within an evacuated outer jacket.

The high velocity chargedA particles may be arranged to issue from the apparatus through a suitable window or in some cases the particles may be arranged to strikea suitable target contained within' the apparatus. Thus X-rays of high energy may be generated by allowing a' stream of highly accelerated electrons to strike a target electrode within the apparatus.

The invention will be further described with reference to two forms of it illustrated i'n the accompanying drawings, in which Figure 1 il: lustrates schematically apparatus in which a stream of highly accelerated electrons is adapted to emerge from the apparatus through an electron permeable window, Fig. 2 shows' a section on the line 2-2 in Fig. l looking from the right, and' Figure 3 shows how the apparatus' of Figuge l may be adaptedforX-ray generation.

In Figs. 1 and 2 the apparatus comprises a metal outer jacket l adapted to be exhausted by' the connection of an exhaust tube 2 to an exhaust system.

Within the cylindrical jacket l are supported coaxially three centrally apertured cylindrical hollow resonators 3, 4 and 5 which are joined by. tubesr 6 and 1. coaxial with the resonators. On' it's side remote'from 4, the resonator 3 has fixed to it a tube 8, coaxial with the resonators, which isterminated by a re-entrant glass seal Q which supports4 athermionic cathode lu and heater H.

Within-f the tubel 8 is fixed a diaphragm I2 which serves as an accelerating electrode for the:electrons,emitted from the cathode I0 when the latter. is maintained negative to earth in operation; iiexible leads I3, I4 for the cathode l0 arid'heater l l pass through a further glass seal l5 closing a tubular projection I6 of the jacket? I'.- The cathode lilll and diaphragm* l2, togetherfwith the adjacent'walliof the'.l resonator 3; constitute an electron gun a'ssembly'by'means of? which'- a convergent stream of electrons may be injected into the resonator 3v along its. axis.

To the end? wall of the resonator. 5f is -fixed coaxiall'y'; aA tube Iflfwhich faces an. electronlperm'e'ablel" Window |81' formed'in the` jacket I and through which electrons from! theY cathode IU :are: adapted' to emerge' after undergoing suc- A'ing centrally across the waveguide, thefcoupling loo'p'sl extending through cooperating apertures in the abutting wallof the waveguide and resonators'. 'I-'he loops' 2-5, 26, 21= are spaced effectiv'ely'athalfewavelengths apart' along th'ewave- -guide I9; the distance-betweenthe' loop 2-1 andv the. closed end 2U- being' effectively al quarterlength': The resonators' 3, 4; 5l are of! progressively' increasing axial? length'- and are' arranged so.' thatthe' position of the coupling loopsrelativeto the axial centre ofthe resonators varies so as'.l to* ensure! the' correct relative phase of excitation of the` resonators.

Around' thevja'ckret' I5- in the region of the' res-` onators are: fitted magnetic focussingcoils 31' for producing" aV magnetic held along the' axis of the resonators which prevents the electron stream! from' spreading unduly.

Thel exact dimensions of' th'e' systeml required, including the axial length of' the'resonators, the position and spacing of the coupling loops, and the' amount` of attenuation required in they attelldatingY waveguide" Sect'b 23- Will depend u'pOn th'ely nature 'of the' acceleration required, on the characteristics of the magnetron and waveguide used an'di in the operating frequency, and must be finally determined b y t'rialfor each case.

For'gui'dan'c'efit may be saidthat for operation with alv magnetron tuned to a wavelength ofl 10 cms., the resonators should'be chosen to-resonate atla' frengencyf about 10 megacycles per second higher thanlthatof the magnetron; the internal diameter of the cylindrical resonators may be about 3 inches and the ,exact resonant frequency required secured by adjustment of tuning plugs 3l, 32, 33.

If the rectangular waveguide yI9 has sides 1 inch by 4 inches, the effective wavelength along the guide will be 12.1 cm., so that the distance between the coupling points 25--26 and 2li-21 along the guide should each vbe 6 .05 cms. and the distance between the coupling-point 21 and the closed end 20 should be 3.025 cms.

If the electrons are injected into the resonator 3 by the electron gun with an energy of about 46 kilovolts, the axial lengths of the resonators 3, 4, 5 may be about 3.5 cms., 4 cms. and 5.2 cms. respectively with the lengths of the tubes 6 and 'l being 0.7 cm. and 0.6 cm. respectively.

The points where the coupling loops 25, 26, 21 pass through the aperture in the abutting walls of the resonators and waveguide I9 may then be distant about 0.9 cm., 2.8 cms., and 4.2 cms. respectively from the end face of the resonator nearer the electron gun, but it will be appreciated that these distances will depend on many factors, such as the size and shape of the coupling loops and the energy fed into the resonators so that the optimum distances must be determined by trial for each particular case; the figures quoted are for a magnetron pulseoperated with 2 micro-second pulses at a repetition rate of 500 per second, the power in each pulse reaching the resonators being about 1.4 megawatts. After three accelerations at the resonators the electrons may reach an energy of about 2 to 3 x 10 electron volts.

I'he apparatus may, after nal assembly and adjustment, be evacuated and sealed olf via the tube 2 or it may be arranged to be continually exhausted through the tube 2 in operation.

Figure 3 shows the modification of Figure 1 required to adapt the apparatus of Figure 1 for X-ray generation, the electron permeable window I8 not being present and the end wall 36 of the envelope supporting instead, via watercooling means 34, a target electrode 35 arranged to .be struck by the highly accelerated electrons emerging from the tube l1, the X-rays thereby generated passing out through the end wall 36.

In either embodiment of the invention the resonators 3, 4, 5 may also be arranged to be water-cooled if required.

I claim:

1. Apparatus for accelerating electrons to energy exceeding 1,000,000 electron volts, comprising an electron gun for producing a stream of electrons along an axis, a plurality of hollow resonators, each resonant at the same frequency, spaced along said axis and each disposed to produce when excited a high frequency alternating electric eld having a component along said axis, a generator of high frequency energy for exciting said resonators, and a wave-guide for conveying said high frequency energy to said resonators, the distance apart along the waveguide of the coupling points to said resonators being greater than the distance between the corresponding resonators along said axis for compensating for the excess of the phase velocity of the exciting energy along the wave-guide over the velocity of the electrons along said axis to produce progressive acceleration of the electrons.

2. Apparatus for accelerating electrons to energy exceeding 1,000,000 electron volts, comprisingy an electron gun for producing a stream of electrons along an axis, a plurality of hollow resonators, each resonant at the same frequency, spaced along said axis and each disposed to produce when excited a high frequency alternating electric field having a component along said axis, a generator of high frequency energy for exciting said resonators, and a wave-guide for conveying the exciting energy from said generator to said resonators, the couplings from said Waveguide to said resonators being spaced apart along the wave-guide at a distance electrically equivalent to one half-wave length of the oscillations in the wave-guide, and the distance between the centres of said resonators along said axis being less than this equivalent half-wave length and increasing progressively to maintain synchronism of excitation of the resonators with the electron stream for progressive acceleration of the electrons.

3. Apparatus for accelerating electrons to energy exceeding 1,000,000 electron volts, comprising an electron gun for producing a stream of electrons along an axis, a plurality of cylindrical hollow resonators coaxial with and spaced along said axis and each resonant at the same frequency, the axial length of successive resonators increasing progressively away from the electron gun and the resonators being apertured axially for the passage of the electron stream, a generator of high frequency energy for exciting said resonators and a wave-guide for conveying the exciting energy from said generator to said resonators, the couplings from said waveguide to said resonators being spaced apart along the wave-guide at a distance electrically equivalent to one half-wave length of the oscillations in the wave-guide, and the distance between the centres of said resonators along said axis being less than the equivalent half-wave length and increasing progressively to maintain synchronism of excitation of the resonators with the electron stream for progressive acceleration of the electrons.

4. Apparatus according to claim 3 wherein the `electron gun and hollow resonators are contained within an evacuated enclosure which is provided with an electron-permeable window for the emergence of the highly accelerated electrons.

5. Apparatus according to claim 3 comprising an electron intercepting electrode lying across the axis of the electron stream on the side of the resonators away from the electron gun so as to be struck by the highly accelerated electrons for the generation of X-rays.

6. Apparatus according to claim 3 wherein the said generator of high frequency energy comprises a resonant cavity magnetron.

'7. Apparatus for accelerating charged particles to very high velocity comprising means for producing a stream of the particles along an axis, a plurality of hollow resonators, each resonant at the same frequency, spaced along said axis and each disposed to produce when excited a high frequency alternating electric field having a component along said axis, a generator of high frequency energy for exciting said resonators, and a wave-guide for conveying said high frequency energy to said resonators, the distance apart along the wave-guide of the coupling points to said resonators being greater than the distance between the corresponding resonators along said axis for compensating for the excess of the phase velocity of the exciting 1. nergy' along the wave-guides over: the velocity; of L the;` particles 1 along saidi axisl` to f produce Vpro;-y griessve acceleration` of'. the` particles.

WILLIAM vERNEST WILLSHAW.

REFERENCES I CITED f Thefollowing references are.- of record i'nthev lof this patent:

UNITED-STATES PATENT-s Number Nme y Date McArthurY Mar. 18,' 1911 Varian et ali Mar. 10,3 1942' Brown Apr; 14, 1942 zwory-kin Jury 14,11942 Gibson- Apr. 20', 1943v AlfordL Aug.v 5; 1947 Hansen et' al. Feb. 1, 1949 Webster Fb. 27, 19'5'1 

